Fluorinated polymers are known in the art. Fluorinated polymers are generally known for their chemical and thermal stability. Said thermal and chemical stability is however negatively affected by the presence of ionic end-groups in the polymeric chain, for instance —CH2OH, —COF, —COOH type end-groups. For instance, —COF and —COOH type end-groups are known to initiate the so-called “unzipping reaction”, wherein starting from these end-groups, the main chain of the fluorinated polymer progressively disaggregates following the reaction scheme sketched here below:Rf—CF2COOH+.OH→Rf—CF2.+CO2+H2ORf—CF2.+.OH→Rf—CF2OH→Rf—COF+HFRf—COF+H2O→Rf—COOH+HFwherein Rf represents the fluorinated polymer chain.
Several methods have been proposed in the past aiming at improving the stability of fluorinated polymers by reduction of the number of unstable end-groups.
GB 1210794 (E.I. DUPONT DE NEMOURS AND COMPANY) Oct. 28, 1970 discloses a process for the stabilization of high molecular weight fluorocarbon polymers in the solid state (as particulate or pre-molding form or as molded articles) by contacting said polymers with a fluorine radical generating compound (e.g. gaseous fluorine) in the absence of oxygen.
U.S. Pat. No. 4,743,658 (E.I. DUPONT DE NEMOURS AND COMPANY) May 10, 1988 discloses a process for the stabilization of tetrafluoroethylene/perfluoroalkylvinylether copolymers by fluorination of the same under the form of pellets by solid/gas reaction with fluorine gas.
These processes however generally require the use of high temperature to obtain the full conversion of the unstable-end groups.
Also it is known in the art a process for the stabilization of amorphous polymers. EP 1256591 A (AUSIMONT S.P.A.) Nov. 13, 2002 and EP 1256592 A (AUSIMONT S.P.A.) Nov. 13, 2002 disclose a process for the stabilization of amorphous perfluorinated polymers, wherein the polymer is first dissolved in a suitable solvent so as to obtain a solution having a concentration of from 0.5 to 15% by weight, and then said solution is submitted to fluorination with elemental fluorine in the presence of UV radiation. Fluoropolymers stabilized according to such method are endowed with a substantial absence of unstable polar end groups, i.e. undetectable by FT-IR spectroscopy.
GB 1226566 (MONTECATINI EDISON SPA) Apr. 4, 1967 discloses a process for the removal of ionic end-groups in perfluorinated polyethers which comprises fluorination of the perfluorinated polyether in a liquid phase with fluorine at a temperature of 100 to 350° C.
It has now been found that the reduction of unstable end-groups can be efficiently performed on a fluorinated polymer under mild reaction conditions and with high conversion of the unstable end-groups by treating said fluorinated polymer with fluorine in the presence of a (per)haloolefin.